The present invention relates to fans and, more particularly, to an improved cowl for the latter and its application to motorised fan units for example for vehicles.
In numerous industrial fields, it is necessary to combine a fan with a radiator or other type of heat exchanger. This is, for example, the case in the motor industry where motorised fan units are fitted to the radiators used for cooling the heat engine for driving vehicles or heat exchangers used for example for air conditioning of the passenger compartment of vehicles.
As is known, it is not easy to control the flow streams of the air agitated by a fan helix so as to improve its efficiency, at constant air flow rate, without thereby increasing the energy necessary for driving the helix of the fan and also without noticeably increasing the sound level during operation.
Various attempts have been made. Some relate to the profile of the blades of the helix, others relate to the cowl of the helix made, therefore, from a stationary part connected to the radiator or to the exchanger and a moving part connected to the blade tips of the fan.
One solution of the first type is, for example, illustrated by the document FR 2,603,953 corresponding to U.S. Pat. No. 4,739,077 . This document proposes giving the blades of the helix a profile such that the thin streams of air close to the hub which carries the roots of the blades describe centripetal trajectories, whereas the thin streams of air closest to the tips of the blades follow centrifugal trajectories. One solution of the other type is, for example, illustrated by the document EP 0,026,997 corresponding to U.S. Pat. No. 4,329,946 . This document proposes using a cowl for a suction fan, placed downstream of the radiator in the direction of flow of the air. This cowl comprises a stationary part or diffuser with a cylindrical skirt and a moving part with a peripheral profiled annular hoop which is rigidly connected to the tips of the blades of the helix and which is mounted coaxially with this skirt so as to be able to rotate therein. In order to attempt to prevent disturbing countercurrents which circulate between skirt and hoop from downstream to upstream, it is arranged that the air streams closest to the blade tips are ejected so as to make a fluid barrier which prevents the circulation of such disturbing countercurrents. It will nevertheless be observed that this solution which, in principle, prevents undesirable back flow of agitated fluid, necessitates a relatively significant clearance between the skirt and the hoop.